The present invention relates to an electro-optical modulator suitable for use in optically recording data.
One known laser beam generator for use as a laser beam source for optically recording data comprises a semiconductor laser, a laser medium, and a nonlinear optical device. A laser beam generated by the semiconductor laser is applied to excite the laser beam to emit a laser beam which is introduced as a fundamental into the nonlinear optical device, and a shorter-wavelength laser beam is obtained as a second harmonic from the nonlinear optical device. The laser beam generator has a low power requirement and is capable of producing a high laser beam power density. Use of the laser beam generator as a laser beam source for optically recording data requires an external beam intensity modulator for modulating an output laser beam that is emitted from the laser beam generator. The external beam intensity modulator has to meet the following requirements:
It has to have a wide modulation frequency band;
It must not cause optical damage at a laser beam intensity high enough to optically record data;
The extinction ratio, i.e., the ratio of the minimum to the maximum value of an output laser beam intensity, must be large;
It must have a low drive voltage, i.e., a half-wavelength voltage; and
The loss of an incident laser beam must be small.
Conventional external beam intensity modulators include an acousto-optical modulator and an electro-optical modulator. The acousto-optical modulator has drawbacks in that its modulation frequency band is narrow, i.e., it is of 40 MHz, and it causes a large loss of incident laser beam, i.e., it causes a loss of 30%. Since the electro-optical modulator has a higher modulation rate of 500 MHz or more and a smaller loss of incident laser beam than the acousto-optical modulator, studies have been directed to efforts to use the electro-optical modulator in applications to optically record data.
One conventional electro-optical modulator will be described below with reference to FIG. 1 of the accompanying drawings. The electro-optical modulator has a pair of electro-optical crystals 1, 2 each of LiTaO.sub.3 or LiNbO.sub.3. Each of these electro-optical crystals 1, 2 is in the form of a rectangular parallelepiped cut in the directions of "a", "b", and "c" crystal axes. The electrooptical crystal 1 has parallel planes 1a, 1a' confronting each other, parallel planes 1b, 1b' confronting each other and extending perpendicularly to the planes 1a, 1a', and parallel planes 1c, 1c' confronting each other and extending perpendicularly to the planes 1a, 1a' and the planes 1b, 1b'. The parallel planes 1a, 1a' are of a face "a", the parallel planes 1b, 1b' are of a face "b", and the parallel planes 1c, 1c' are of a face "c". Similarly, the electro-optical crystal 2 has parallel planes 2a, 2a' confronting each other, parallel planes 2b, 2b' confronting each other and extending perpendicularly to the planes 2a, 2a', and parallel planes 2c, 2c' confronting each other and extending perpendicularly to the planes 2a, 2a' and the planes 2b, 2b'. The parallel planes 2a, 2a' are of a face "a", the parallel planes 2b, 2b' are of a face "b", and the parallel planes 2c, 2c' are of a face "c".
Each of the electro-optical crystals 1, 2 has a length of 2 mm along the "a" axis, a length of 12 mm along the "b" axis, and a length of 2 mm along the "c" axis. The faces "b" 1b, 1b', 2b, 2b' of the electro-optical crystals 1, 2 are polished to an optical grade, and coated with a nonreflective coating layer which does not reflect a laser beam used. The "c" axes of the electro-optical crystals 1, 2 are tilted 90.degree. with respect to each other for thereby removing natural birefringence and preventing an output laser beam intensity from being varied by temperature fluctuations.
Electrodes 3, 4 are deposited on the entire faces "c" 1c, 1c', 2c, 2c' of the electro-optical crystals 1, 2 by evaporation. A signal source (oscillator) 9 has terminals connected respectively to the electrodes 3 of the electro-optical crystal 1 and also connected respectively to the electrodes 4 of the electro-optical crystal 2 for applying a signal voltage to the electro-optical crystals 1, 2 in the direction of the "c" axis.
A collimated polarized laser beam 7 which has a polarized direction (plane of polarization) at 45.degree. with respect to the "a" and "c" axes of the electro-optical crystal 1 is applied perpendicularly to a central region of the face "b" 1b of the electro-optical crystal 1. The collimated polarized laser beam 7 passes through the electro-optical crystals 1, 2, and is emitted from the face "b" 2b' of the electro-optical crystal 2. The emitted laser beam travels through a polarizer (analyzer) 10 and is applied to a light detector 11. The polarizer 10 is positioned such that its direction of polarization (plane of polarization) extends perpendicularly to the polarized direction (plane of polarization) of the laser beam 7 that is applied to the face "b" 1b of the electro-optical crystal 1. When a signal voltage from the signal source 9 is applied between the electrodes 3, 4 of the electrooptical crystals 1, 2, the refractive indexes of the electro-optical crystals 1, 2, are varied depending on the applied signal voltage, rotating the polarized direction (plane of polarization) of the laser beam 7. Since the laser beam 7 with its polarized direction (plane of polarization) rotated is applied through the polarizer (analyzer) 10 to the light detector 11, the light detector 11 produces a detected signal having a level commensurate with the signal voltage from the signal source 9.
The conventional electro-optical modulator shown in FIG. 1 suffers the following problems:
(1) The conventional electro-optical modulator has a large halfwave voltage. The halfwave voltage is inversely proportional to the distance between the electrodes on the electro-optical crystals. In order to reduce the halfwave voltage, it is necessary to employ a very slender, long crystal. However, it is difficult to produce such a crystal according to the ordinary grinding process. It is difficult to reduce the halfwave voltage of the conventional electro-optical modulator shown in FIG. 1 to 100V or lower.
The halfwave voltage is a voltage applied to an electro-optical modulator to vary the optical phase difference between two orthogonal linearly polarized components by .pi. (rad.) due to an electro-optical effect. Inasmuch as the laser beam intensity can be modulated by a modulation degree of 100% with the halfwave voltage, the halfwave voltage is used as one of indications of the quality of the electro-optical modulator and the optically modulating material. Because the halfwave voltage depends upon the configuration of the device, it is indicated as being standardized by its value at the time the optical path length and the interelectrode distance are equal to each other so that the halfwave voltage is used as an indication of the quality of the optically modulating material without being affected by the configuration of the device. For phase modulation, the definition of the halfwave voltage may be stretched to mean an applied voltage to given a phase change of .pi. (rad.) to a polarized component in question. The above explanation of the term "halfwave voltage" is in accord with "Dictionary of Optical Terms" published Nov. 30, 1981 by Ohm Co., Ltd.
(2) The conventional electro-optical modulator tends to cause optical damage. In order to obtain a high extinction ratio, it is necessary to reduce the beam diameter of a laser beam in an electro-optical crystal. If the beam diameter of a laser beam in an electro-optical crystal is reduced, then the optical density in the electro-optical crystal is increased. If the electro-optical crystal is made of LiTaO.sub.3 or LiNbO.sub.3, then when a laser beam of a reduced diameter which is intensive enough to optically record data, the optical density exceeds an optical damage threshold, and hence causes optical damage to the electro-optical crystal, which is thus unable to be used as the electro-optical modulator.
Generally, the optical damage is a phenomenon in which a solid body is damaged by the application of an intensive laser beam. Specifically, a solid body is irreversibly damaged on its surface or in its internal structure by a temperature rise, a plasma generation, a self convergence, or a stimulated Brillouin effect. In a narrower sense, the optical damage also signifies a reversible phenomenon in which a carrier excited by light is moved in LiNbO.sub.3 or the like and caught by a trap, generating an internal electric field which causes a refractive index to be locally varied by an electro-optical effect. The above definition of the optical damage is given by the dictionary referred to above.
(3) The extinction ratio of the conventional electro-optical modulator is poor. The extinction ratio is degraded if the beam diameter of a laser beam applied to an electro-optical crystal is increased to increase the optical density.
The extinction ratio is the ratio Imin/Imax (or its reciprocal) of the minimum value Imin to the maximum value Imax which the intensity of output light can take in the intensity modulation of a light wave, and represents one of indications of the quality of an optical modulator. The extinction ratio is important particularly in the field of digital modulation. Reduction of a code transmission error rate requires a small extinction ratio. This definition of the term is also given by the dictionary referred to above.